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November 10, 1991

SWITCHED MULTI-MEGABIT DATA SERVICES AND ASYNCHRONOUS TRANSFER MODE: ALLIES OR RIVALS?

By CBR Staff Writer

Two new industry groupings coalesced towards the end of Telecom ’91 in Geneva – the ‘ATM Forum’ (nothing to do with Automatic Teller Machines) aims to produce, a standard interface for network devices using the Asynchronous Transfer Mode protocol. ‘The European SMDS interest group’, on the other hand is hoping to speed the development of the Switched Multi-Megabit Data Service. Taking the Asynchronous Transfer Mode forum first, its four members are Adaptive Corp (a subsidiary of Network Equipment Technology Corp), Northern Telecom Ltd, Cisco Systems Inc, and US Sprint Communications Co. According to reports from the show, Alcatel NV, Apple Computer Inc and Pacific Telesis Group Inc’s Pacific Bell may follow suit. Asynchronous Transfer Mode is the switching technology used to provide broadband ISDN services (B-ISDN), the high-speed communications technology for carrying voice, data and video. Switched Multi-megabit is also a high-speed service, but runs over the IEEE 802.6 Metropolitan Area Network standard and unlike the Asynchronous Transfer Mode offers a connectionless service, rather than connection-oriented one like Asynchronous Transfer Mode and B-ISDN. Alcatel is one of the founders of the European SMDS Interest Group, together with Digital Equipment Corp, Hewlett-Packard Corp, L M Ericsson Telefon AB, Siemens AG and six of the European carriers including British Telecommunications Plc and Televerket of Sweden. There are differences between the two technologies of course: although 802.6 is capable of carrying isochronous traffic (traffic that like voice or video requires guaranteed real-time delivery) implementing isochronous services is more complicated than asynchronous data. This, combined with the marketing decision that the world did not need another way to transmit voice as a priority, led the US Baby Bells to implement Switched Multi-megabit as a data only service. Asynchronous Transfer Mode is also designed to run at higher speeds starting at 155 Mbps as compared with SMDS running at between 1.5Mbps and 45Mbps. The relationship between the two technologies depends on who you talk to: SMDS is likely to be the first to make it into the real world – the standards are relatively stable and 802.6 does not such substantial rebuilding of the communications infrastructure as full-blown Asynchronous Transfer Mode network. Some, then see it as an interim solution, to be replaced by B-ISDN, others such as Hewlett-Packard’s SMDS project manager Stanley Ooi, believe that the long term future is secure due to the interoperability of the two standards. Each Asynchronous Transfer Mode packet contains virtual channel identifiers, used to track the data’s source and eventual destination, since SDMS is a connectionless protocol any data passing from an SMDS across the Asynchronous Transfer Mode will not have this information present. The problem is solved by giving 802.6 traffic a special virtual channel identifier. When an Asynchronous Transfer Mode switch encounters a packet labelled in this way, it passes it to a separate connectionless server Asynchronous Transfer Mode transport network node. The switch should be ready within three years.

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